1. Field of the Invention
The present invention relates to a vibration actuator, and, more particularly, to a vibration actuator that is capable of minimizing change of natural frequency due to manufacturing tolerance of raw materials to improve vibration characteristics of the vibration actuator, simplifying and reducing a manufacturing process to improve productivity of the vibration actuator, and improving durability to increase the service life of the vibration actuator.
2. Description of the Related Art
Generally, a mobile communication device, such as a mobile phone or a pager, incorporates a vibration actuator that is capable of individually or simultaneously outputting sound or vibration. The vibration actuator outputs a voice signal, which is electrically or electronically received, or a previously inputted bell or melody as audible sound, or resonates at a specific frequency to output an incoming signal as vibration that a person can feel.
FIG. 13 is a cross-sectional view illustrating a conventional vibration actuator 1. As shown in FIG. 13, the conventional vibration actuator 1 comprises a case 1a formed in the shape of a hollow cylinder. The case 1a has an opened lower part, which is closed by a shield plate 2. Also, the case 1a has an opened upper part, at which is securely mounted a diaphragm 3, which is a sound-generation oscillating plate, by a supporting ring 13, which is separately fixed to the case 1a. Specifically, the outer-circumferential part of the diaphragm 3 is securely inserted in the inner wall of the case 1a. To the center of the lower surface of the diaphragm 3 is securely fixed a voice coil 4.
Between the shield plate 2 and the diaphragm 3 is disposed a plate spring 5, which comprises a spring body 5a having an opened center part and a plurality of elastic legs 5b extending from the spring body 5a. The elastic legs 5b are securely fixed to the inner wall of the case 1a. 
To the lower surface of the plate spring 5 is integrally attached an upper surface of a mass member 6. The mass member 6 has a mounting hole 6a formed through the center thereof such that a yoke 7 is inserted in the mounting hole 6a of the mass member and an engaging protrusion 6b extending from the outer circumferential part thereof such that the engaging protrusion 6b is engaged with the inner circumferential surface of the case 1a. 
On the upper surface of the yoke 7, which is inserted in the mounting hole 6a of the mass member 6, is mounted a vertically magnetized magnet member 8. On the magnet member 8 is disposed an upper plate 9. The yoke 7, the magnet member 8, and the upper plate 9 constitute a magnetic field unit 10.
On the upper surface of the shield plate 2 is mounted a vibration coil 11, which is disposed directly under the yoke 7.
When electric current is supplied to the vibration coil 11, the mass member 6, including the magnetic field unit 10, is excited by an elastic force of the plate spring 5 due to interaction between the electric field generated by the vibration coil 11 and the magnetic field generated by the magnetic field unit 10. As a result, the vibration actuator is vibrated.
When electric current is supplied to the voice coil 4, the diaphragm 3 is oscillated due to interaction between the electric field generated by the voice coil 4 and the magnetic field generated by the magnetic field unit 10. As a result, sound is generated from the vibration actuator.
The plate spring 5 provided at the conventional vibration actuator 1 is manufactured by processing a thin metal sheet according to a pressing process and a wire discharging process. During the pressing process and the wire discharging process, minute defects and cracks are generated at the cut surface of the plate spring. As a result, fatigue lifetime of the plate spring 5 is decreased, and therefore, the service life of the vibration actuator 1 is reduced.
When the thickness of the plate spring 5 is changed by approximately 1 μm, stiffness of the plate spring 5 is increased or decreased by 2 gf/mm. Such stiffness change of the plate spring 5 directly affects natural frequency of the plate spring 5. For this reason, it is necessary to strictly control the thickness of the plate spring 5, which elastically supports the entire vibrator, including the mass member 6 and the magnetic field unit 10. However, it is difficult to strictly control the thickness of the plate spring 5 due to the manufacturing process of the plate spring 5, and therefore, it is difficult to uniformly maintain the natural frequency of the vibration actuator 1.
The mass member 6 is attached to the spring body 5a of the plate spring 5 by spot welding while the upper surface of the mass member 6 is in contact with the lower surface of the spring body 5a of the plate spring 5, and ends of the elastic legs of the plate spring 5 are fixed to the inner circumferential surface of the case 1a. 
The natural frequency of the vibration actuator 1 is sharply changed depending upon positions where the spot welding operation between the plate spring 5 and the mass member 6 has been performed, and therefore, the vibration characteristics of the vibration actuator 1 is deteriorated. Furthermore, the manufacturing process of the vibration actuator 1 is complicated, and therefore, productivity of the vibration actuator 1 is decreased.